1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates circuits for optimizing the performance of phase shift keyed modulation schemes.
While the present invention is described herein with reference to a particular embodiment for particular applications, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings of the present invention will recognize additional modifications and applications within the scope thereof.
2. Description of the Related Art
The advantages of phase shift keyed (PSK) modulation are well recognized. PSK modulation schemes are used to allow for efficient power distribution in spread spectrum communications links. PSK is used to encode digital information in time division multiple access systems. PSK pulse coding in radar systems has been found to enhance the performance and range of the system. PSK has recently been employed in navigation systems.
In PSK modulation, an RF signal is modulated by a data code. The data code is typically a train of pulses which vary between +1 and -1. The data code effectively shifts the phase of the RF signal between nominal positive and negative levels. Demodulation therefore requires the removal of the coded or keyed phase shift.
Where the modulation code is periodic or pseudo-random, demodulation at the receiver is relatively straightforward. All that is required is the mixing of a synchronized duplicate of the modulation code with the received modulated signal. However, where the code is random ie. data, another demodulation scheme must be used.
Such schemes typically provide for capacitive AC coupling of a signal into a balanced mixer. In digital applications, AC coupling works well with suitably high data rates. As the data rates become low, however, the quality of the signal provided by the coupling capacitor to the mixer may become unsatisfactory. The corresponding undesirable effect on the mixer output may result in errors in the received data.
In addition, it is known that when the output of the mixer is maintained at a constant level, problems associated with reception are minimized. The design of the receiver should be optimized and matched with the design of the transmitter. To achieve these these advantages, DC coupling has been considered as an alternative.
DC coupling (or level shifting) is often accomplished with linear amplifiers. While eliminating the problem with low data rates suffered by AC coupled schemes, DC coupling is problematic in that: (1) linear amplifiers often provide an output that is sensitive to power supply variations and (2) low power linear amplifiers are typically not fast enough and high speed devices typically consume too much power for space applications.
In Analysis and Design of Analog Integrated Circuits, (1977) by P. Gray and R. Meyer, p. 241; a circuit as shown which is relatively insensitive to power supply variations. The circuit is a current source. No means are shown for switching the current source to provide a high speed bipolar output.
Thus there is considered to be a need in the art for a high speed DC coupled mixer driver for a PSK modulator that is insensitive to power supply variations with minimal power consumption.